This invention relates to a method for enhanced oil recovery from underground oil-bearing formations. More particularly, this invention relates to a dissolution equipment for producing a stable aqueous solution of polyacrylamide powder for use in enhanced oil recover.
In general, the so-called primary method of oil recovery has been practiced for many years as a method for recovering oil from underground oil reservoirs and comprises finding underground oil-reservoirs by exploration of oil, establishing oil recovery plants and recovering crude oil by free flowing or pumping until an economical oil recovery rate is obtained. Recently, the techniques for exploring oil reservoirs have been greatly developed, whereby it has become possible to analyze the state of oil reservoirs more exactly. Together with this development a method for enhanced oil recovery which comprises sweeping crude oil by injecting a fluid into underground oil-bearing formation which are in a state of decreased oil recovery after primary recovery has been conducted widely.
The fluid to be used in such methods for enhanced oil recovery is generally water, and thus such methods have been called "water flooding". In this connection, an aqueous solution of a water-soluble polymer has been sometimes used as the fluid thereby to control the mobility of the fluid in the oil-bearing formations. Polyacrylamide has been popularly used as a desirable polymer.
The polyacrylamide to be used for this purpose should be of an ultra-high molecular weight. One of the methods for obtaining the aqueous solution thereof is to dissolve in water the polyacrylamide powder, which is ordinarily packaged in a paper bag or the like and delivered to consumers. However, the operation of dissolving the polymer is not always easy because of reasons such as the ultra-high molecular weight of the polyacrylamide and the necessity of the resulting solution to withstand severe conditions in the oil reservoir. More specifically, polyacrylamide powder has hitherto been dissolved in water by opening bags containing it, pouring the powder into a hopper, supplying the powder to a dispersion vessel by means of a screw-type feeder, and dissolving the powder by agitation to produce an aqueous solution thereof.
This procedure has been accompanied by several problems, however, such as contamination upon opening the bags by, for example, chips of the materials of bags, contamination with oxygen present in the voids among the powder, formation of lumps due to moisture, inconstant supply due to formation of bridge and flash which are often encountered in a screw-type feeder, and formation of lumps and poor dispersion in liquid due to upward blowing (for lack of a means for blowing gas downwards). Thus, the aqueous polymer solution thus prepared is totally unfit for the mobility-controlling fluid because of contamination of foreign substances, deterioration due to oxygen, formation of insolubles, and unstable concentration of polymer, as well as other reasons. In fact, when the resulting solution or a solution obtained by dilution thereof to a desired concentration was injected into an underground oil-bearing formation, it has been often found that the polymer does not function as desired, and also plugging is caused to stop production of crude oil.
Although all of these problems are serious, precaution should be especially paid to the deterioration due to oxygen when an aqueous solution of polyacrylamide is used for sweeping oil. More specifically, the oil reservoirs where the polymer solution works are often under a high temperature and pressure, and thus the deterioration of the aqueous solution of polyacrylamide injected thereinto is markedly caused to occur when oxygen is dissolved in the solution. As a result, the viscosity of the solution is substantially decreased to a level as low as that of the injection water, whereby the purpose for using a viscous solution cannot be achieved.
In general, oxygen is not present in reservoirs. As long as the aqueous solution is prepared as described above, however, it is difficult to avoid the presence of the dissolved oxygen which causes such deterioration of polymer solution in the reservoirs. Even when an attempt is made to maintain the system for preparing the solution under a nitrogen gas atmosphere, it is not easy to apply the nitrogen atmosphere to all of the equipment units. Even if such a modification is possible, the oxygen in the voids among polyacrylamide powder cannot be removed by simply placing the system for producing the solution under a nitrogen gas atmoshere.
Furthermore, the steps of metering, supply and dissolution are not always easy because the substance to be dissolved is polyacrylamide powder. Since the polyacrylamide powder is apt to undergo caking when compressed, the metering and supply which cause compression (for example by means of a screw-type feeder) are not suitable as mentioned above. Because caking of polyacrylamide power readily takes place, especially when it absorbs moisture, the metering and supply systems accompanying compression which are placed before the dissolution step are especially unfit for this purpose. Furthermore, a homogeneous solution cannot be obtained with formation of "lump" if water is added to the polyacrylamide powder for dissolution thereof, unless extremely vigorous agitation is conducted.